The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
The human ability to spatially localize a sound source is to a large extent dependent on perception of the sound at both ears. Due to different physical distances between the sound source and the left and right ears, a difference in time of arrival of a given wavefront of the sound at the left and right ears is experienced (the Interaural Time Difference, ITD). Consequently, a difference in phase of the sound signal (at a given point in time) will likewise be experienced and in particular perceivable at relatively low frequencies (e.g. below 1500 Hz). Due to the shadowing effect of the head (diffraction), a difference in level of the received sound signal at the left and right ears is likewise experienced (the Interaural Level Difference, ILD). The attenuation by the head (and body) is larger at relatively higher frequencies (e.g. above 1500 Hz). The detection of the cues provided by the ITD and ILD largely determine our ability to localize a sound source in a horizontal plane (i.e. perpendicular to a longitudinal direction of a standing person). The diffraction of sound by the head (and body) is described by the Head Related Transfer Functions (HRTF). The HRTF for the left and right ears ideally describe respective transfer functions from a sound source (from a given direction) to the ear drums of the left and right ears. If correctly determined, the HRTFs provide the relevant ITD and ILD between the left and right ears for a given direction of sound relative to the user's ears. Such HRTFleft and HRTFright are preferably applied to a sound signal received by a left and right hearing assistance device in order to improve a user's sound localization ability (cf. e.g. Chapter 14 of [Dillon; 2001]).
Several methods of generating HRTFs are known. Standard HRTFs from a dummy head can e.g. be provided, as e.g. derived from Gardner and Martin's KEMAR HRTF database [Gardner and Martin, 1994] and applied to sound signals received by left and right hearing assistance devices of a specific user. Alternatively, a direct measurement of the user's HRTF, e.g. during a fitting session can—in principle—be performed, and the results thereof be stored in a memory of the respective (left and right) hearing assistance devices. During use, e.g. in case the hearing assistance device is of the Behind The Ear (BTE) type, where the microphone(s) that pick up the sound typically are located near the top of (and often, a little behind) pinna, a direction of impingement of the sound source is determined by each device, and the respective relative HRTFs are applied to the (raw) microphone signal to (re)establish the relevant localization cues in the signal presented to the user.
WO2010115227A1 describes a method and system for allowing a recipient of a bilateral hearing aid system, to locate the source of a sound signal about the recipient using localization cues (e.g. interaural level difference (ILD)) in the sound signal, which are modified (e.g. transposed to a lower frequency) to provide useable localization cues to generate a stimulating signal for application to the recipient.
US2004218771A1 describes a method for producing an approximated partial transfer function for use in an electroacoustic appliance for producing an environment correction transfer function that matches an appliance transfer function for the electroacoustic appliance to an acoustic environment. An environment correction transfer function to standard HRTFs is determined, the correction determining the partial transfer function due to the specific environmental conditions and the particular hearing aid used.
WO2009001277A1 describes a binaural object-oriented audio decoder for providing an enhanced binaural object-oriented audio decoder by modifying the received head-related transfer function parameters according to a received desired distance, which allows an arbitrary virtual positioning of objects in a space.